Method for controlling driving of inkjet head, and inkjet recording apparatus

ABSTRACT

There is provided a method for controlling driving of an inkjet head including recording elements each including a nozzle and a driving element. The method includes a pulse width setting step. In this step, when a predetermined number of ink droplets ejected according to the predetermined number of driving pulses are made to land in the same pixel range, the predetermined number of first driving pulses each having a pulse width longer than a reference pulse width and the predetermined number of second driving pulses each having a pulse width shorter than the reference pulse width are combined for each of the recording elements and obtained combinations are respectively output to the recording elements. The predetermined number is two or more.

TECHNICAL FIELD

The present invention relates to a method for controlling the driving ofan inkjet head and an inkjet recording apparatus.

BACKGROUND ART

There is a technique for changing the density gradation of each pixelrange by causing a plurality of continuously ejected ink droplets toland in the same pixel range, either in combination in the middle orseparately, in an inkjet recording apparatus that forms a desired image,structure, thin film, and the like on a medium by ejecting ink dropletsfrom nozzles. In the inkjet recording apparatus, there are variations inink ejection characteristics between nozzles. In particular, whenejecting a plurality of ink droplets continuously, the influence of theprevious ejection operation tends to affect the subsequent ejectionoperation. For this reason, the variation tends to be large andcomplicated.

In order to reduce the influence of the variation, there is a techniqueof adjusting an electrical signal (driving pulse) for driving a drivingelement, which applies a pressure fluctuation to ink in a nozzle, foreach driving element. Patent Literature 1 discloses a technique ofadjusting the fall timing of the driving waveform in the driving pulseof each driving element to match the amount of ink droplets and thelanding timing each other.

CITATION LIST Patent Literature

-   Patent Document 1: JP 2017-226201A

SUMMARY OF THE INVENTION Technical Problem

Conventionally, however, inkjet heads with variations in ink ejectioncharacteristics above the standard have been discarded as non-standardproducts. As the number of nozzles increases and demands for inkejection characteristics increase, there is a problem that the yield ofinkjet heads decreases, leading to an increase in cost, and the like.

It is an object of the present invention to provide a method forcontrolling the driving of an inkjet head and an inkjet recordingapparatus capable of obtaining an inkjet head that can be used withreduced variations in ejection characteristics in a wider range.

Solution to Problem

In order to achieve the aforementioned object, an invention describedclaim 1 is a method for controlling driving of an inkjet head includinga plurality of recording elements each including a nozzle through whichink is ejected and a driving element that applies a pressure fluctuationto ink supplied to the nozzle according to an applied driving pulse. Areference pulse width of the driving pulse that maximizes apredetermined characteristic value related to ink droplets ejected byeach of the recording elements according to the driving pulse applied tothe driving element with respect to a change in pulse width of thedriving pulse has a variation equal to or greater than a predeterminedstandard. The driving control method includes a pulse width setting stepin which, when a predetermined number (two or more) of ink dropletsejected according to the predetermined number of driving pulses are madeto land in the same pixel range, the predetermined number of firstdriving pulses each having a pulse width longer than the reference pulsewidth and the predetermined number of second driving pulses each havinga pulse width shorter than the reference pulse width are combined foreach of the plurality of recording elements and obtained combinationsare respectively output to the plurality of recording elements.

According to an invention described in claim 2, in the driving controlmethod described in claim 1, in the pulse width setting step, the firstdriving pulse having a pulse width longer than any of the referencepulse widths related to the plurality of recording elements and thesecond driving pulse having a pulse width shorter than any of thereference pulse widths are set.

According to an invention described in claim 3, in the driving controlmethod described in claim 2, in the pulse width setting step, the pulsewidth of the first driving pulse and the pulse width of the seconddriving pulse are commonly set for the plurality of recording elements.

According to an invention described in claim 4, in the driving controlmethod described in any one of claims 1 to 3, in the pulse width settingstep, an order of the first driving pulse and the second driving pulseis set such that a pulse width closer to the reference pulse widthcorresponding to a minimum one of the maximum characteristic valuesrelated to the plurality of recording elements becomes a last drivingpulse.

According to an invention described in claim 5, in the driving controlmethod described in any one of claims 1 to 4, the predeterminedcharacteristic value is a droplet speed of ejected ink.

According to an invention described in claim 6, in the driving controlmethod described in any one of claims 1 to 4, the predeterminedcharacteristic value is an amount of ink droplets to be ejected.

According to an invention described in claim 7, in the driving controlmethod described in any one of claims 1 to 6, the predetermined standardfor the variation is 3%.

According to an invention described in claim 8, in the driving controlmethod described in any one of claims 1 to 7, the predetermined numberis an even number, and in the pulse width setting step, the firstdriving pulse and the second driving pulse are set so as to bealternately output.

An invention described in claim 9 is an inkjet recording apparatusincluding: an inkjet head including a plurality of recording elementseach including a nozzle through which ink is ejected and a drivingelement that applies a pressure fluctuation to ink supplied to thenozzle according to an applied driving pulse; and a control unit thatcontrols an output of the driving pulse applied to the driving elementto each of the recording elements. A reference pulse width of thedriving pulse that maximizes a predetermined characteristic valuerelated to ink droplets ejected by each of the recording elementsaccording to the driving pulse applied to the driving element withrespect to a change in pulse width of the driving pulse has a variationequal to or greater than a predetermined standard. When a predeterminednumber (two or more) of ink droplets ejected according to thepredetermined number of driving pulses are made to land in the samepixel range, the control unit combines the predetermined number of firstdriving pulses each having a pulse width longer than the reference pulsewidth and the predetermined number of second driving pulses each havinga pulse width shorter than the reference pulse width with each other foreach of the plurality of recording elements and outputs obtainedcombinations to the plurality of recording elements, respectively.

Advantageous Effects of Invention

According to the present invention, there is an effect that it ispossible to more easily reduce variations in ejection characteristicsbetween nozzles.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view showing the schematic configuration of aninkjet recording apparatus.

FIG. 2 is a bottom view showing a bottom surface of a head unit facing atransport belt.

FIG. 3 is a block diagram showing the functional configuration of aninkjet recording apparatus.

FIG. 4A is a diagram for explaining an ejection pulse.

FIG. 4B is a diagram for explaining an ejection pulse.

FIG. 5A is a diagram showing an example of a driving waveform when inkis continuously ejected multiple times.

FIG. 5B is a diagram showing an example of a driving waveform when inkis continuously ejected multiple times.

FIG. 6A is a diagram showing an example of the distribution of theejection speeds of a plurality of nozzles in the inkjet head.

FIG. 6B is a diagram showing an example of the distribution of theejection speeds of a plurality of nozzles in the inkjet head.

FIG. 7A is a diagram showing an example of the ejection speeddistribution when the amount of change in pulse width is changed.

FIG. 7B is a diagram showing an example of the ejection speeddistribution when the amount of change in pulse width is changed.

FIG. 8A is a diagram for explaining variations in sensitivity betweennozzles.

FIG. 8B is a diagram for explaining variations in sensitivity betweennozzles.

FIG. 9 is a diagram showing an example of the distribution of ejectionspeeds according to the order of pulse widths when nozzles havingdifferent sensitivities are included.

FIG. 10 is a flowchart showing the control procedure of a drivingwaveform setting process performed by the inkjet recording apparatus.

DETAILED DESCRIPTION

Hereinafter, an embodiment of the present invention will be describedwith reference to the diagrams.

FIG. 1 is a perspective view showing the schematic configuration of aninkjet recording apparatus 1 according to the present embodiment.

The inkjet recording apparatus 1 includes a transport unit 10, arecording operation unit 20, a control unit 40, an image capturing unit50, and the like.

The transport unit 10 moves a medium M on which the image is to berecorded, and ejects the medium M through an image recording position.The transport unit 10 includes a driving roller 11, a transport belt 12,a driven roller 13, a transport motor 14, a pressing roller 15, and thelike.

The transport belt 12 is endless and is stretched between the drivingroller 11 and the driven roller 13, and moves as the driving roller 11rotates. The driving roller 11 rotates at a speed according to therotation of the transport motor 14. The driven roller 13 rotates at aspeed according to the movement of the transport belt 12. An image isrecorded on the medium M while being moved in a state in which themedium M is placed at a predetermined position on the outer peripheralsurface of the transport belt 12, and the medium M is discharged at apredetermined position after the image is recorded. The pressing roller15 presses the medium M placed on the transport belt 12 against thetransport belt 12 to remove lifting of the medium M due to wrinkles orthe like. The pressing roller 15 may press the medium M against thetransport belt 12 by its own weight and rotate according to the movementof the medium M and the transport belt 12.

The recording operation unit 20 has a plurality of nozzles that ejectink onto the medium M on the transport belt 12, and records an imageaccording to the timing and amount of ink ejected from each nozzle.Although not particularly limited, the recording operation unit 20includes a head unit 21C that ejects cyan ink, a head unit 21M thatejects magenta ink, a head unit 21Y that ejects yellow ink, and a headunit 21K that ejects black ink. That is, the recording operation unit 20can eject four colors of ink. Hereinafter, some or all of these are alsoreferred to as a head unit 21.

The control unit 40 centrally controls the overall operation of theinkjet recording apparatus 1. The control unit 40 will be describedlater.

The image capturing unit 50 images the surface of the transport belt 12(medium M placed thereon) on the downstream side of the recordingoperation unit 20 in the transport direction of the medium M on thetransport belt 12 by the transport unit 10. The image capturing unit 50is, for example, a line sensor in which a CCD (Charge-Coupled Device)imaging element or a CMOS (Complementary Metal Oxide Semiconductor)imaging element is arranged in the width direction, and two-dimensionalimaging on the medium M is possible in combination with the movement ofthe medium M in the transport direction.

FIG. 2 is a bottom view showing a surface (bottom surface) of the headunit 21 facing the transport belt 12. Since the head units 21C, 21M,21Y, and 21K have the same structure, only one of these will bedescribed herein.

The head unit 21 is fixed to a carriage 210. Nozzle surfaces of 16inkjet heads 211 provided in the head unit 21 are exposed to the bottomsurface of the head unit 21. A large number of nozzle openings 27 a arearranged on each nozzle surface. The openings 27 a are arranged atpredetermined intervals (nozzle pitches) in the width direction so thatthe ejected ink lands at each position in the width direction on themedium M being transported.

FIG. 3 is a block diagram showing the functional configuration of theinkjet recording apparatus 1.

The inkjet recording apparatus 1 includes a driving waveform signalgenerating unit 29, a storage 30, a communication unit 70, an operationreceiving unit 81, a display unit 82, and a power supply unit 90, andthe like in addition to the transport unit 10, the recording operationunit 20, the control unit 40, and the image capturing unit 50 describedabove.

The transport unit 10 includes the transport motor 14 as describedabove, and outputs an appropriate driving signal to the transport motor14 to rotate the transport motor 14.

The recording operation unit 20 includes a head driving unit 25, apiezoelectric element 26 (driving element), and the like. The headdriving unit 25 applies a driving signal (driving pulse) to the selectedpiezoelectric element 26 to deform the piezoelectric element 26. As aresult, the piezoelectric element 26 applies a pressure fluctuationcorresponding to the driving pulse to the ink supplied to a nozzle 27 tocause the ink to be ejected from the nozzle 27, thereby recording animage. The piezoelectric element 26 and the nozzle 27 form a recordingelement 200 according to the present embodiment. The recording operationunit 20 has a plurality of recording elements 200 corresponding to thenumber of nozzles 27.

The driving waveform signal generating unit 29 generates a driving pulseoutput from the head driving unit 25 to the recording element 200.Although not particularly limited, the driving waveform signalgenerating unit 29 converts digital data indicating a predetermineddriving waveform into analog data, and outputs a signal obtained byamplifying the voltage and current to the head driving unit 25 as adriving pulse.

The control unit 40 is a processor that includes a CPU 41 (CentralProcessing Unit) and a RAM 42 (Random Access Memory) and performsoverall control of various operations of the inkjet recording apparatus1. The CPU 41 performs various kinds of arithmetic processing to performcontrol operations. The RAM 42 provides a working memory space for theCPU 41, and stores temporary data. The control unit 40 controls theoutput of driving pulses related to the ink ejection operation of theinkjet head 211 to the recording element 200 based on image data to berecorded, setting data related to image recording, and the like.

The storage 30 stores image data to be recorded, and also stores variousprograms or setting data. The storage 30 may have at least anon-volatile memory, or may have a volatile memory (RAM). The image datamay be stored in the RAM. The setting data includes AL (Acoustic Length)measurement data 31 and waveform setting data 32. The non-volatilememory is, for example, a flash memory and the like and may additionallyor alternatively have an HDD (Hard Disk Drive) and the like.

The AL measurement data 31 stores the actual AL (reference pulse width)measurement value related to the ink inside each nozzle 27 (including anink flow path communicating with the nozzle 27). The AL is half theresonance period (acoustic resonance period) of pressure vibrationoccurring in the ink (fluid) in the nozzle 27. The AL depends on thestructure of the nozzle 27 and the like, that is, the length, width, andthe like of the nozzle 27. Since the nozzle and the ink flow pathcommunicate with a common ink supply path on the further upstream side,the AL may deviate slightly from the theoretically accurate value. thereis a slight variation in the structure due to manufacturing, and the ALalso varies slightly according to the variation. The AL measurement data31 does not need to include the ALs of all the nozzles 27, and may besampled at predetermined intervals or the like, or ALs obtained bypartially narrowing the interval may be stored as necessary.

The waveform setting data 32 stores waveform pattern data of drivingpulses to be output to each recording element 200. The waveform patterndata stored herein particularly includes information of the starttiming, pulse width, and voltage amplitude of the driving pulsecorresponding to each ink ejection when a plurality of ink ejections arecontinuously performed. These may be digital data that is a source ofthe driving pulse generated by the driving waveform signal generatingunit 29.

The communication unit 70 performs and controls communication with anexternal device. For example, the communication unit 70 is connected toan external computer based on a communication standard such as TCP/IP,and can acquire job data including image data to be recorded and outputthe status of the image recording operation based on the job data. Thecommunication unit 70 may be directly connected to a peripheral devicethrough a USB (Universal Serial Bus) or the like to transmit and receivedata.

The operation receiving unit 81 receives an input operation by a user orthe like, and outputs the received content to the control unit 40 as aninput signal. The operation receiving unit 81 includes, for example, atouch panel, a push button switch, or the like. The touch panel may belocated so as to overlap the display screen of the display unit 82, andthe operation content may be specified in synchronization with thedisplay content on the display screen.

The display unit 82 displays such as a status and a selection menu forthe user and the like. The display unit 82 has, for example, a displayscreen and an indicator (lamp), and the like. The display unit 82 has,for example, a liquid crystal display, and the like and can displayvarious characters or graphics on the display screen in a dot matrix.The indicator may be used, for example, to indicate the presence orabsence, or the like of power supply, or the presence or absence of anoperational abnormality with an LED lamp or the like.

The power supply unit 90 supplies power of a voltage corresponding toeach unit of the inkjet recording apparatus 1. A voltage correspondingto the peak voltage of each driving waveform is output to a drivingsubstrate 212 of the recording operation unit 20. Alternatively, onlythe maximum peak voltage may be output to generate a plurality ofvoltage signals at the driving substrate 212.

In addition to the above components, the inkjet recording apparatus 1may include a measurer that measures the speed of ejection of ink fromeach nozzle 27. Alternatively, the inkjet recording apparatus 1 mayinclude a mounting portion for an external measurement device formeasuring the speed of ejection of ink from each nozzle 27. The ejectionspeed may be determined from the landing position based on image datacaptured by the image capturing unit 50 instead of directly measuringthe flying speed of the ink.

Next, driving settings related to the ink ejection operation in theinkjet recording apparatus 1 according to the present embodiment will bedescribed.

FIGS. 4A and 4B are diagrams for explaining ejection pulses.

As shown in FIG. 4A, the ink ejection operation herein is performed byapplying a pressure fluctuation to the ink through an operation ofapplying a driving pulse of trapezoidal wave (or rectangular wave) tothe piezoelectric element 26 so that the ink supplied to the nozzle 27is temporarily compressed or expanded in the ink flow path (ink chamber)in front of the nozzle 27 and then returning the ink to the previousstate. For the sake of explanation, the rise from the initial voltage ofthe voltage of the trapezoidal wave and the fall to the initial voltageare shown with easy-to-understand lengths herein. However, the rise timeand fall time compared to the period during which the driving voltage ismaintained may be set appropriately.

As for the ejection of the ink from the nozzle 27, in the case ofpushing for compressing the ink first, the ink pushed out from theopening 27 a by compressing the ink flow path flies by being separatedfrom the ink in the ink flow path that is pulled back by the operationof returning to the original volume of the ink flow path. In the case ofpulling for expanding the ink first, the ink is pulled back from theopening 27 a of the nozzle 27 to the back of the flow path by expandingthe ink flow path is returned to the direction of the opening 27 a ofthe nozzle 27 powerfully by the operation of returning to the originalvolume of the ink flow path. As a result, a part of the ink at thedistal portion jumps out from the opening 27 a to be separated and fly.

In this pressure fluctuation, the ink has a large vibration componentwith a period corresponding to the above-described AL. By applying adriving pulse with a pulse width (here, the time from the start of therise to the start of the fall of the driving pulse in the trapezoidaldriving wave is assumed to be a pulse width Pw) of AL, the kineticenergy of the ink is efficiently obtained from the driving pulse.

As shown in FIG. 4B, the ejection speed (characteristic value) decreases(changes) according to the applied pulse width, in particular, as thepulse width largely deviates from the actual AL for the nozzle 27. Theejection speed can be approximated by, for example, a quadratic curve ora cubic curve (or a higher-order function) with respect to the amount ofdeviation of the pulse width from the actual AL. In accordance with thedeviation of the AL between the plurality of nozzles 27 (here, two typesare illustrated by the thick line and the thin line), the position ofthe approximation curve also deviates. Usually, if the ink ejectionspeed at a predetermined representative value Pw0 does not deviate morethan the standard between the nozzles 27, the inkjet head 211 having thenozzles 27 can be used. As the representative value Pw0, for example,the AL of a nozzle located at the center in the arrangement of thenozzles 27 is selected.

FIGS. 5A and 5B are diagrams showing examples of a driving waveform whenink is continuously ejected multiple times.

As shown in FIG. 5A, in the inkjet recording apparatus 1, the inkdensity (gradation) of each pixel range is set in multiple stages bycombining the ink (multi-drop ink) that is continuously ejected multipletimes (a predetermined number of two or more) during flight or by makingthe ink (multi-drop ink) that is continuously ejected multiple timesland within a predetermined pixel range on the medium M. In a case wherea pulse width Pw1 is equal to the AL, when the ejection operation isperformed multiple times at the ejection period Pe1 that is twice thepulse width Pw1, the vibration is amplified (resonating) by thereverberation of the amplitude of the ink related to the previousejection operation during the second and subsequent ejections. As aresult, the ejection speed increases. On the other hand, when the pulsewidth Pw1 is not equal to the AL or when the ejection period Pe1 is nottwice the pulse width Pw1 or the AL, the ejection speed may not increaseduring the second and subsequent ejections, and furthermore, thevibration may be weakened to decrease the ejection speed. That is,compared with a case of single-shot ejection, in continuous ejectionperformed multiple times, variations in ejection speed may increaseaccording to the relationship between the set pulse width and the actualAL of each nozzle.

In the inkjet recording apparatus 1 according to the present embodiment,for example, it is possible to use the inkjet head 211 in whichvariations in the reference pulse width (actual AL) at which the inkdroplet speed (predetermined characteristic value related to inkdroplets) takes a maximum value with respect to changes in the pulsewidth, that is, the ink droplet speed (predetermined characteristicvalue related to ink droplets) at the actual AL may be so large that itcannot be ignored in terms of image quality (above a predeterminedstandard according to image quality). Alternatively, a standard, such aswhen the droplet ejection speed of a driving pulse having a pulse widthof the representative value Pw0 (AL of the central nozzle) describedabove is above a predetermined standard, may be used for convenience. Inthe inkjet recording apparatus 1, when continuously performing ejectionand landing multiple times (especially an even number of times) in thesame pixel range (including a case where ink droplets are combined inthe middle), a first pulse width Pw1 is made different from a secondpulse width Pw2 as shown in FIG. 5B. Problems in image quality due tovariations in ejection speed include, for example, deviation of inklanding position, instability of ink droplet flight due to too low inkspeed, penetration of ink into the medium M at the time of landing, andvariations in spread and fixation, and the like. Since the amount ofdeviation of the ink landing position depends on the movement speed(transport speed) of the flying medium M and the like, the predeterminedstandard cannot be uniformly set. However, for example, the deviationamount of the ink landing position may be set based on the maximummovement speed of the medium M that can be executed by the inkjetrecording apparatus 1. In the current inkjet recording apparatus 1, thestandard for the amount of deviation of the landing position (dropletspeed) can be set to 3% or the like, for example. In a case where thepredetermined standard may or may not be satisfied depending on thenumber of continuous ejections, the transport speed, and the like(collectively, the operating conditions), when the predeterminedstandard is not satisfied under at least one of the conditions, theinkjet head 211 may be controlled to make the first pulse width Pw1 andthe second pulse width Pw2 different from each other uniformlyregardless of the operating conditions. That is, it is not necessary toswitch the setting of each driving pulse according to the operatingconditions.

As described above, when the pulse width of the applied driving pulsedeviates greatly from the actual AL, the vibration associated with theprevious ejection tends to weaken the vibration associated with the nextejection. In this case, the ejection speed decreases significantly. Ifthe later ejected ink is slower than the previously ejected ink, the inkdroplets that are supposed to be combined may not be combined duringflight. If the speed difference is too large, problems may occur notonly in the deviation of the landing position of the ink but also in theshape of the ink droplets or the manner in which the ink droplets land(that is, the image quality and the like may be degraded). For thisreason, a plurality of ink droplets should be within the proper speedrange.

In the inkjet recording apparatus 1 according to the present embodiment,the pulse width and the like are set by combining two consecutivedriving pulses. It is assumed that one of the two driving pulses is adriving pulse (first driving pulse) having the pulse width Pw1 longerthan the ALs (including the AL of the nozzle itself through which theink is ejected) of all the nozzles in one head unit 21 to be adjustedand the other one is a driving pulse (second driving pulse) having thepulse width Pw2 shorter than the ALs (including the AL of the nozzleitself through which the ink is ejected) of all the nozzles in the headunit 21. That is, all the driving pulses are not set as long as the ALfor any nozzle. On the other hand, by making the lengths of the twotimes different, a nozzle to which two pulses having a small lengthdifference from the AL is supplied and a nozzle to which two pulseshaving a large length difference from the AL is supplied are notgenerated. This combination of driving pulses may be commonly set as adriving pulse for all nozzles.

FIGS. 6A and 6B are diagrams showing examples of the distribution of theejection speeds of a plurality of nozzles (here, 100 nozzles) in theinkjet head 211. In the following description, the ejection speedrepresents the speed after the ink droplets continuously ejectedmultiple times are combined. However, when the speed is calculated basedon the imaging result of the image capturing unit 50, an average speedfrom the ejection timing of any ink droplet (for example, the lastdroplet) to the landing on the medium M may be calculated.

As indicated by a line Lk1 in FIG. 6A, when the ejection operation(here, two ejections) for one pixel is performed with a fixed pulsewidth Pwm (for example, 8.6% shorter than the representative value Pw0),nozzles with nozzle numbers of 55 and later have a high ejection speed(after a plurality of droplets are combined), and nozzles with nozzlenumbers of 45 and before have a low ejection speed. On the other hand,as indicated by a line Lk2, when the same ejection operation isperformed with a fixed pulse width Pwp (>Pwm, for example, 8.6% longerthan the representative value Pw0), the nozzles with nozzle numbers of45 and before have a higher ejection speed than that in the case of thepulse width Pwm, and the nozzles with nozzle numbers of 55 and laterhave a lower ejection speed than that in the case of the pulse widthPwm. That is, it is estimated that the AL is close to the pulse widthPwp for the nozzles with nozzle numbers of 45 and before and the AL isclose to the pulse width Pwm for the nozzles with nozzle numbers of 55and later.

Lines Lj1 and Lj2 respectively show an ejection speed at each nozzlewhen the pulse width Pw1 is sufficiently shorter than the pulse widthPwm in the first ejection operation of the two ejection operations andthe pulse width Pw2 is sufficiently longer than the pulse width Pwp inthe second ejection operation. Sufficiently herein means short enough tobe shorter than the ALs of all the nozzles and long enough to be longerthan the As of all the nozzles, respectively. Unless there is a clearabnormality in the nozzle, even if the ALs of all the nozzles are notactually measured and acquired, the range of AL variation can be roughlyassumed based on some measurement results and manufacturingcharacteristics. Therefore, the pulse widths Pw1 and Pw2 may be set in arange (here, ±15.2% of the representative value Pw0) that is larger thanthe assumed range of variation. The pulse width satisfies therelationship Pwm+Pwp=Pw1+Pw2. In the line Lj1, the ejection period istwice the pulse width Pwm, and in the line Lj2, the ejection period istwice the pulse width Pwp. In both the cases, the variation in ejectionspeed is smaller than the variation in the AL of each nozzle, ascompared with a case of the fixed pulse width.

Although not particularly limited, for example, the ejection period Pe1is equal to or less than twice the pulse width Pw2 and equal to orgreater than twice the pulse width Pw1. For example, the ejection periodPe1 may be about twice as long as the average AL of all the nozzles, ormay be twice the pulse width Pw1 or the pulse width Pw2.

Such a combination of pulse widths is not limited to the case of twoejections, and may be applied to a case of four ejections, for example.In the case of continuously ejecting ink four times or more, a set oftwo driving pulses may be repeated twice or more. By alternatelyoutputting driving pulses having long and short pulse widths, variationsin overall ejection speed are reduced for nozzle rows having portionswith different ALs.

Not only when performing ejection continuously four times or more for asingle pixel range but also in the case of high-frequency ejection, suchas when the ink ejection to the next pixel range is started before thereverberation related to the ink ejection to the previous pixel rangedisappears, the variation in the ejection speed tends to increasebecause the influence of the vibration of the ink related to theprevious ejection influences the vibration of the ink related to thesubsequent ejection.

In FIG. 6(b), lines Li1 and Li2 show the distribution of the inkejection speed for the first pixel and the distribution of the inkejection speed for the tenth pixel, respectively, when four inkejections per pixel are performed with a pulse width Pwc (Pwp>Pwc>Pwm,where the pulse width Pwc is 5.7% shorter than the representative valuePw0). The recording period of each pixel (pixel range) is approximately5.22 times the ejection period. In this case, for nozzles (nozzlenumbers of 55 and later) whose ink ejection speed is close to the ALwhen recording the tenth pixel, the ink ejection speed for the firstpixel decreases due to the reverberation of the waveform between pixels.That is, the ejection speed changes depending on the state of continuousoperation.

Lines Lv1 and Lv2 show the distribution of the ink ejection speed forthe first pixel and the distribution of the ink ejection speed for thetenth pixel when the waveforms having pulse widths P1 a and P2 a (P1 a(=0.686×Pw0)<Pwc<P2 a (=1.286×Pw0)) are alternately switched betweenodd-numbered and even-numbered ejections, respectively. It can be seenthat not only is the variation for the first pixel smaller than thatindicated by the line Li1, but also the difference between the inkejection speed distribution for the tenth pixel and the ink ejectionspeed distribution for the first pixel is smaller than that when thepulse width Pwc is fixed. In this manner, a driving signal in which along pulse width and a short pulse width are combined is also effectivefor stability during continuous operation.

A combination of pulse widths that reduces the variation in the relativeejection speed does not necessarily result in conversion to the mostefficient kinetic energy (ejection speed). Therefore, a desired ejectionspeed (absolute value) can be obtained by adjusting the amplitude(voltage) of the driving waveform.

It is necessary to adjust the pulse width from the representative valuePw0 according to the difference in AL for each nozzle when the influenceof the variation in AL between nozzles on the image quality is so largethat it cannot be ignored as described above. According to the highesttransport speed of the medium M or the highest level of the requiredimage quality, for example, when there is a variation of 3% or more inthe ejection speed or the amount of ink droplets, it may be necessary toadjust the pulse width from the representative value Pw0 according tothe difference in AL for each nozzle. As described above, the pulsewidths Pw1 and Pw2 are set to intervals equal to or larger than theinterval between the maximum value and the minimum value of the ALvariation (for example, about 10%) according to the ejection speedvariation. As a result, for example, the pulse widths Pw1 and Pw2 mayincrease or decrease by about ±20% (40% in total) with respect to theconventionally set representative value Pw0 of the pulse width, or mayincrease or decrease even more than this.

FIGS. 7A and 7B are diagrams showing examples of the ejection speeddistribution when the amount of change in pulse width is changed.

In FIG. 7A, the distribution (lines Lra and Lrb) of the ejection speedwhen the ejection operation is performed four times for one pixel withthe fixed pulse width Pwc is compared with the distribution (lines Laaand Lab) of the ejection speed when the second and fourth pulse widthsare extended by about 9%. In the lines Lra and Laa, the ejection periodis twice the pulse width Pwc, and in the lines Lrb and Lab, the ejectionperiod is twice the second and fourth pulse widths. This level ofdifference in amplitude does not result in a large change, but there isa tendency that the variation is slightly reduced compared with a caseof the fixed pulse width.

In FIG. 7B, the first and third pulse widths are shortened by about 18%from the pulse width Pwc, and the second and fourth pulse widths areextended by about 27% from the pulse width Pwc. The ejection periods forthe lines Lba and Lbb respectively are the same as those for the linesLaa and Lab. In these cases, there is a tendency that the variation inejection speed between nozzles is more effectively reduced.

The piezoelectric element 26 corresponding to each nozzle has variationsin sensitivity.

FIGS. 8A and 8B are diagrams for explaining variations in sensitivitybetween nozzles.

For example, as shown in FIG. 8A, there are a nozzle whose ejectionspeed is maximized at the pulse width Pwa and a nozzle whose ejectionspeed is maximized at the pulse width Pwb, and their maximum ejectionspeeds Va and Vb are different. That is, even if driving pulses having apulse width equal to the actual AL are applied, the same ejection speedmay not be obtained.

In the example shown in FIG. 8B, when ejection speeds from a pluralityof nozzles in the inkjet head 211 are measured at the representativevalue Pw0 of the pulse width, nozzles with nozzle numbers of 0 to 25 andnozzles with nozzle numbers after 50 apparently have relatively highejection speeds as indicated by a line Lt. However, when the ejectionspeeds of the plurality of nozzles are measured at the pulse width Pwmshorter than the representative value Pw0, as indicated by a line Ls,the ejection speeds of nozzles with nozzle numbers of 60 and later tendto further increase, and the ejection speeds of nozzles with nozzlenumbers before 50 tend to decrease. On the other hand, when the ejectionspeeds of the plurality of nozzles are measured at the pulse width Pwplonger than the representative value Pw0, as indicated by a line Lu, theejection speeds of nozzles with nozzle numbers of 55 and later decrease,and the ejection speeds of nozzles with nozzle numbers before 50increase. That is, it can be seen that the AL of the nozzles with nozzlenumbers of 55 and later is shorter than the representative value Pw0 andthe AL of nozzles with nozzle numbers before 50 is longer than Ps.However, for nozzles with nozzle numbers of 25 to 45, even if thenozzles are driven with the pulse width Pwp close to the AL, theirejection speeds are not greatly different from the ejection speeds ofthe nozzles with nozzle numbers after 50. That is, it can be seen thatthe sensitivity of the piezoelectric element 26 itself is low for thenozzles with nozzle numbers of 25 to 45.

In such a case, in the two sets of driving pulses described above, theorder of pulse widths is set such that the pulse width of the second(last) driving pulse is closer to the AL (reference pulse width) of thenozzle with low sensitivity (minimum one of the maximum characteristicvalues (ejection speeds)). When the sensitivity is originally low andthe speed tends to be low, the ink tends to land properly as a whole byrelatively increasing the speed of the subsequent ink so that thesubsequent ink catches up with the leading ink.

FIG. 9 is a diagram showing an example of the distribution of ejectionspeeds according to the order of pulse widths when nozzles havingdifferent sensitivities are included. The sensitivity of nozzles(piezoelectric elements 26) with nozzle numbers before 50 is relativelylow, and the sensitivity of nozzles (piezoelectric elements 26) withnozzle numbers after 50 is relatively high.

A line Lh shows a difference between the reference ejection speed andthe ejection speed when the first pulse width is closer to the actual ALfor nozzles with nozzle numbers before 50 and the second pulse width iscloser to the actual AL for nozzles with nozzle numbers after 50. A lineL1 shows a difference between the reference ejection speed and theejection speed when the first pulse width is closer to the actual AL fornozzles with nozzle numbers after 50 and the second pulse width iscloser to the actual AL for nozzles with nozzle numbers after 50. It canbe seen that the difference in ejection speed indicated by the line L1is smaller than the difference in ejection speed indicated by the lineLh.

Next, a pulse width setting operation in the inkjet recording apparatus1 according to the present embodiment will be described.

As described above, setting the pulse width is not necessary if the ALvariation within the head unit 21 is sufficiently small, and isperformed instead of discarding the head unit 21 when the AL variationis large. Since the AL at each nozzle 27 deviates from the theoreticalvalue, the ejection speed is measured while shifting the pulse width toperform fitting, and the AL at each nozzle 27 is specified as beingequal to the pulse width (reference pulse width) that maximizes theejection speed by the fitting. The distribution of the AL within thehead unit 21 (inkjet head 211) may be estimated based on a predeterminednumber of measurement data, as described above.

FIG. 10 is a flowchart showing the control procedure of a drivingwaveform setting process, which is performed by the inkjet recordingapparatus 1 according to the present embodiment, by the control unit 40.The driving waveform setting process is started, for example, inresponse to an input operation on the operation receiving unit 81 by aninspector at the time of inspection before shipping.

When the driving waveform setting process is started, the control unit40 (CPU 41) sets a plurality of (for example, three) pulse widths and aplurality of nozzles 27 for measuring the ink ejection speed. Thecontrol unit 40 sets all combinations of the pulse widths and nozzlepositions, and causes the recording operation unit 20 to eject ink withthe set nozzles 27 and pulse widths and causes a measurer to measure theink ejection speed (step S101).

The control unit 40 performs fitting of the obtained speed distributionfor each nozzle 27 whose ink ejection speed is to be measured (forexample, fitting by the quadratic curve or the cubic curve describedabove) to estimate the maximum speed and the pulse width (referencepulse width) at the maximum speed (step S102). The maximum speed can beconverted into sensitivity information of the piezoelectric element 26of the nozzle 27. The reference pulse width is associated with the AL(actual AL) related to the nozzle 27.

The control unit 40 estimates the maximum and minimum values of theactual AL based on the distribution of the actual AL in the nozzlearrangement direction (step S103). The maximum and minimum values do nothave to be exact values, and may be estimated at wider intervals (largermaximum value and smaller minimum value). For this estimation, thecontrol unit 40 may additionally acquire measurement data of the inkejection speed at the plurality of pulse widths for some nozzles 27. Inaddition, the control unit 40 may use fitting or the like to estimatethe maximum and minimum values.

The control unit 40 sets a fixed pulse width and an ejection period Pebased on the estimated AL of each nozzle (step S104). The fixed pulsewidth may be the conventionally set representative value Pw0, but is notlimited thereto. The ejection period Pe may be twice the fixed pulsewidth.

The control unit 40 determines whether the variation in the ink ejectionspeed of all the nozzles 27 at the set fixed pulse width falls within apredetermined standard (step S105). For example, the control unit 40calculates how much the ink ejection speed from the nozzle 27 with theAL farthest from the fixed pulse width (either the maximum value or theminimum value of the AL; Both may be mechanically selected) is lowerthan the ink ejection speed at the AL. If it is determined that thevariation in the ink ejection speed of all the nozzles 27 at the setfixed pulse width falls within the predetermined standard (“YES” in stepS105), the control unit 40 determines the set fixed pulse width andejection period Pe, and ends the driving waveform setting process.

If it is determined that the variation in the ink ejection speed of allthe nozzles 27 at the set fixed pulse width does not fall within thepredetermined standard (“NO” in step S105), the control unit 40 sets thepulse widths Pw1 and Pw2 and the ejection periods Pe1 and Pe2 based onthe estimated maximum and minimum values of the AL (step S106). Thepulse widths Pw1 and Pw2 may be obtained, for example, by multiplyingthe median value (average value) of the maximum and minimum values ofthe AL by a predetermined multiple (for example, 1.2 times and 0.8times, and the like) of the value. The ejection period Pe1 may be, forexample, twice the obtained pulse width Pw1 or Pw2. The ejection periodPe2 may be the same as the ejection period Pe1, for example.

The control unit 40 compares the sensitivity distribution in thearrangement direction (width direction) of the nozzles 27 with the ALdistribution. The control unit 40 determines the order as to which ofthe pulse widths Pw1 and Pw2 is to be output first (step S107). Thecontrol unit 40 specifies a range of relatively low sensitivity amongall the nozzles 27 and acquires a representative AL value in this range.The control unit 40 determines which of the pulse widths Pw1 and Pw2 iscloser to the representative AL, and determines the ejection order sothat the closer one is output later. The processing of steps S106 andS107 forms a pulse width setting step in the driving control methodaccording to the present embodiment. Then, the control unit 40 ends thedriving waveform setting process.

As described above, the inkjet head 211 according to the presentembodiment includes a plurality of recording elements 200 each includingthe nozzle 27 through which the ink is ejected and the piezoelectricelement 26 that applies a pressure fluctuation to the ink supplied tothe nozzle 27 according to the applied driving pulse. The referencepulse width (actual AL) that maximizes the droplet speed of ink dropletsejected by each of the recording elements 200 according to the drivingpulse applied to each of the plurality of piezoelectric elements 26 withrespect to the change in the pulse width of the driving pulse has avariation equal to or greater than a predetermined standard. The methodfor controlling the driving of the inkjet head 211 according to thepresent embodiment includes a pulse width setting step (that is, theprocessing of steps S106 and S107 in the driving waveform settingprocess) in which, when a predetermined number (two or more) of inkdroplets ejected according to the predetermined number of driving pulsesare made to land in the same pixel range, the predetermined number offirst driving pulses each having a pulse width Pw1 longer than thereference pulse width (actual AL) and the predetermined number of seconddriving pulses each having a pulse width Pw2 shorter than the referencepulse width (actual AL) are combined for each of the plurality ofrecording elements 200 and obtained combinations are respectively outputto the plurality of recording elements 200.

According to such a driving control method, in the inkjet recordingapparatus 1 that continuously ejects a plurality of droplets to land onthe same pixel range, even in a case where the variations incharacteristics between the nozzles 27 are large and the effect isparticularly large in a plurality of continuous ejections andaccordingly the standard of the inkjet head 211 is not conventionallysatisfied, the adverse effects of variations can be reduced moreeffectively. Therefore, since the inkjet head 211, which hasconventionally been discarded, can be used, the manufacturing yield canbe increased and the manufacturing cost can be reduced.

In the pulse width setting step, the first driving pulse having a pulsewidth Pw1 longer than any of the reference pulse widths related to theplurality of recording elements 200 and the second driving pulse havinga pulse width Pw2 shorter than any of the reference pulse widths areset. That is, instead of setting the pulse widths Pw1 and Pw2 based onindividual reference pulse widths for each nozzle, the long side pulsewidth Pw1 and the short side pulse width Pw2 are set for the ALs of allnozzles. As a result, the pulse widths Pw1 and Pw2 can be made to beappropriately largely different, and the ejection speed of each nozzlecan be stably obtained.

In the pulse width setting step, the pulse width Pw1 and the pulse widthPw2 are commonly set for the plurality of recording elements 200. Bycommonly setting the pulse widths Pw1 and Pw2 satisfying the conditionsfor all the nozzles 27, it is possible to easily perform pulse widthsetting. In addition, since there is no need to change the pulse widthfor each piezoelectric element 26, the driving operation is also easy.

In the pulse width setting step, the order of the first driving pulseand the second driving pulse is set such that a pulse width closer tothe reference pulse width (AL) corresponding to a minimum one of themaximum ink ejection speeds related to the plurality of recordingelements 200 becomes the last driving pulse. That is, for thepiezoelectric element 26 with low sensitivity, a driving pulse close tothe AL is output so that the required ejection speed can be easilyobtained with the last driving pulse. As a result, stable ink ejectionand operating efficiency with respect to the driving voltage can beobtained without lowering the ejection speed more than necessary.

The predetermined characteristic value is a droplet speed of ejectedink. Since the ink droplet speed is likely to greatly affect thevariation in image quality, unevenness in image quality can be easilyreduced by using the ink droplet speed as a characteristic value toadjust the alignment between the nozzles 27.

Alternatively, the predetermined characteristic value may be the amountof ink droplets to be ejected. Depending on the content of an image tobe recorded or the content of a request from a person who records theimage, the unevenness of the ink density (gradation) as a whole may bemore important than the ink landing position. Therefore, the presentinvention is also effective when reducing variations in image quality byusing the amount of droplets as a characteristic value.

The predetermined standard for the allowable range of the variations incharacteristic values is 3%. Although the influence of variationsdepends on the transport speed of the medium M or the required imagequality, a unique appropriate value may be set based on the generallyrequired average image quality or the like. As a result, it is possibleto provide the user of the inkjet recording apparatus 1 with the inkjethead 211, by which an appropriate image quality can be easily and stablyobtained, while simplifying inspection and setting work.

The predetermined number related to continuous ejections of ink dropletsto a single pixel range is an even number. In the pulse width settingstep, the first driving pulse and the second driving pulse are set so asto be alternately output. As a result, it becomes easier to obtain thefinal ink ejection speed for each nozzle in a well-balanced manner.

The inkjet recording apparatus 1 according to the present embodimentincludes: the inkjet head 211 including a plurality of recordingelements 200 each including the nozzle 27 through which ink is ejectedand the piezoelectric element 26 that applies a pressure fluctuation tothe ink supplied to the nozzle 27 according to the applied drivingpulse; and the control unit 40 that controls the output of the drivingpulse applied to the piezoelectric element 26 to the recording element200. In the inkjet recording apparatus 1, the reference pulse width(actual AL) that maximizes the droplet speed of ink droplets ejected byeach of the recording elements 200 according to the driving pulseapplied to each of the plurality of piezoelectric elements 26 withrespect to the change in the pulse width of the driving pulse has avariation equal to or greater than a predetermined standard. When apredetermined number (two or more) of ink droplets ejected according tothe predetermined number of driving pulses are made to land in the samepixel range, the control unit 40 combines the predetermined number offirst driving pulses each having a pulse width Pw1 longer than thereference pulse width (AL) and the predetermined number of seconddriving pulses each having a pulse width Pw2 shorter than the referencepulse width (AL) with each other for each of the plurality of recordingelements 200 and outputs obtained combinations to the plurality ofrecording elements 200, respectively.

According to such an inkjet recording apparatus 1, the non-standardinkjet head 211, which has conventionally been discarded, can be used asbeing capable of ejecting ink at a stable ink ejection speed. Therefore,the inkjet recording apparatus 1 can perform a stable image recordingoperation while reducing the manufacturing/maintenance cost.

The present invention is not limited to the embodiment described above,and various modifications can be made.

For example, although the common driving signal is output to all therecording elements 200 in the inkjet head 211 in the embodimentdescribed above, a driving signal may be set separately or for eachgroup obtained by dividing the recording elements 200 in the inkjet head211 into several groups. In this case, for the recording element 200 towhich a certain driving signal is output, a longer pulse width Pw1 and ashorter pulse width Pw2 than any of the ALs of the recording element 200may be set respectively.

In the embodiment described above, when the variation in the ejectionspeed due to the driving signal whose pulse width is the representativevalue Pw0 exceeds the standard in at least one of the operatingconditions, a driving signal based on a combination of the first drivingpulse and the second driving pulse is always output for two or morecontinuous ink ejections per pixel from the corresponding inkjet head211. However, switching to the operation to output a driving signalbased on a combination of the first driving pulse and the second drivingpulse may occur only under the conditions exceeding the standard.

In the embodiment described above, the ejection speed of ink droplets isused as a characteristic value, but the present invention is not limitedthereto. For example, the amount of ink droplets and the like can alsobe used as a characteristic value.

In the embodiment described above, the ejection speed measured at aplurality of pulse widths is fitted with a quadratic or cubic function,and the maximum value is used as the reference pulse width (AL).However, the reference pulse width (AL) may be directly obtained byperforming measurement at sufficiently narrow intervals near the maximumvalue.

The order of ejection according to the sensitivity of the piezoelectricelement 26 may not be taken into consideration, or one of the longerdriving pulse and the shorter driving pulse may always be preceded andthe other may always follow the one driving pulse.

The specific configuration, the content and procedure of the processingoperation, and the like shown in the above-described embodiment can beappropriately changed without departing from the spirit and scope of thepresent invention. The scope of the present invention includes the scopeof the present invention described in the claims and the equivalentscope thereof.

INDUSTRIAL APPLICABILITY

The present invention can be used for a method for controlling thedriving of an inkjet head and an inkjet recording apparatus.

REFERENCE SIGNS LIST

-   -   1 inkjet recording apparatus    -   10 transport unit    -   11 driving roller    -   12 transport belt    -   13 driven roller    -   14 transport motor    -   15 roller    -   20 recording operation unit    -   21, 21C, 21M, 21Y, 21K head unit    -   25 head driving unit    -   26 piezoelectric element    -   27 nozzle    -   27 a opening    -   29 driving waveform signal generating unit    -   30 storage    -   31 AL measurement data    -   32 waveform setting data    -   40 control unit    -   41 CPU    -   42 RAM    -   50 image capturing unit    -   70 communication unit    -   81 operation receiving unit    -   82 display unit    -   90 power supply unit    -   200 recording element    -   210 carriage    -   211 inkjet head

1. A method for controlling driving of an inkjet head including aplurality of recording elements each including a nozzle through whichink is ejected and a driving element that applies a pressure fluctuationto ink supplied to the nozzle according to an applied driving pulse,wherein a reference pulse width of the driving pulse that maximizes apredetermined characteristic value related to ink droplets ejected byeach of the recording elements according to the driving pulse applied tothe driving element with respect to a change in pulse width of thedriving pulse has a variation equal to or greater than a predeterminedstandard, and the driving control method includes a pulse width settingstep in which, when a predetermined number (two or more) of ink dropletsejected according to the predetermined number of driving pulses are madeto land in the same pixel range, the predetermined number of firstdriving pulses each having a pulse width longer than the reference pulsewidth and the predetermined number of second driving pulses each havinga pulse width shorter than the reference pulse width are combined foreach of the plurality of recording elements and obtained combinationsare respectively output to the plurality of recording elements.
 2. Thedriving control method according to claim 1, wherein, in the pulse widthsetting step, the first driving pulse having a pulse width longer thanany of the reference pulse widths related to the plurality of recordingelements and the second driving pulse having a pulse width shorter thanany of the reference pulse widths are set.
 3. The driving control methodaccording to claim 2, wherein, in the pulse width setting step, thepulse width of the first driving pulse and the pulse width of the seconddriving pulse are commonly set for the plurality of recording elements.4. The driving control method according to claim 1, wherein, in thepulse width setting step, an order of the first driving pulse and thesecond driving pulse is set such that a pulse width closer to thereference pulse width corresponding to a minimum one of the maximumcharacteristic values related to the plurality of recording elementsbecomes a last driving pulse.
 5. The driving control method according toclaim 1, wherein the predetermined characteristic value is a dropletspeed of ejected ink.
 6. The driving control method according to claim1, wherein the predetermined characteristic value is an amount of inkdroplets to be ejected.
 7. The driving control method according to claim1, wherein the predetermined standard for the variation is 3%.
 8. Thedriving control method according to claim 1, wherein the predeterminednumber is an even number, and in the pulse width setting step, the firstdriving pulse and the second driving pulse are set so as to bealternately output.
 9. An inkjet recording apparatus, comprising: aninkjet head including a plurality of recording elements each including anozzle through which ink is ejected and a driving element that applies apressure fluctuation to ink supplied to the nozzle according to anapplied driving pulse; and a hardware processor that controls an outputof the driving pulse applied to the driving element to each of therecording elements, wherein a reference pulse width of the driving pulsethat maximizes a predetermined characteristic value related to inkdroplets ejected by each of the recording elements according to thedriving pulse applied to the driving element with respect to a change inpulse width of the driving pulse has a variation equal to or greaterthan a predetermined standard, and when a predetermined number (two ormore) of ink droplets ejected according to the predetermined number ofdriving pulses are made to land in the same pixel range, the hardwareprocessor combines the predetermined number of first driving pulses eachhaving a pulse width longer than the reference pulse width and thepredetermined number of second driving pulses each having a pulse widthshorter than the reference pulse width with each other for each of theplurality of recording elements and outputs obtained combinations to theplurality of recording elements, respectively.